1. Field of the Invention
Integrated Landing System (ILS) is the landing system currently used worldwide. It operates via the use of paired overlapping fixed beams that define a guidance path. Because of the use of VHF frequencies, i.e., 100 MHz for azimuth guidance and 300 MHz for vertical guidance, the antennas used for ILS are quite large. An azimuth guidance array might be 100 feet wide and a vertical guidance antenna 25 feet high, for example, In addition, the ILS vertical guidance antenna generally requires level smooth terrain for some distance in front of the array since the guidance beam uses the image array (below the smooth surface) to form the guidance beam as is well known in the prior art.
ILS is scheduled to be replaced by MLS (Microwave Landing System), a system whose development was initiated in the 1960's to overcome the then existing deficiencies of ILS. MLS operates at an assigned C-band microwave frequency (5000 MHz). The use of microwave frequencies permits the generation of narrow beams that scan the desired guidance path in both azimuth and elevation. Even though MLS operates at a much higher frequency than ILS, i.e., 5000 MHz rather than 100 MHz and 300 MHz as in ILS, the need to generate narrow scanning beams requires that relatively large, complex and expensive antenna systems be employed. An ILS elevation guidance scanning array assembly, for example, can extend up to 12 feet in height.
In addition to initiating a program in the 1960's to develop MLS as an ILS replacement, the FAA simultaneously embarked on a program to overcome the then existing deficiencies of ILS. As a result of this second program, the operation of ILS has been improved to the point where it now performs very satisfactorily. This has resulted in considerable controversy as to the current merit of replacing ILS by MLS at civil airports.
The MLS program as initiated in the 1960's was primarily a civil effort with the military services during this period independently embarking on various programs to satisfy their landing requirements that could not be satisfied by ILS.
In general, the military's landing system development efforts that occurred during the MLS development period focused on providing simple and very lightweight ground equipment for use in the tactical environment. A basic approach in this connection was to utilize a much higher microwave frequency than that employed by the C-band MLS system in order that the antenna system employed be smaller and, hence, lighter. The type of scanning beam guidance techniques employed by MLS were generally retained, however, the resulting military landing aids that evolved did not result in extremely simple and lightweight ground equipment.
In the late 1970's, a decision was made by the military to generally cease their own landing aid development efforts and to participate in the FAA MLS program. This decision was based on at least two considerations. First was the desire to have a common civil and military landing aid signal format so that military aircraft could use the same airborne receiver at both civil and military airports. Second was the belief that the then on-going MLS program would ultimately yield the desired lightweight ground station. The desired ground station weight for tactical uses was specified in a 1978 A.F. General Operational Requirement (GOR) as being 100 pounds with the equipment capable of being broken down into 35 pound modules for ease in transportation under the tactical environment.
At the present time, as noted above, the MLS program is now facing serious oppositions by civil users, since ILS now generally satisfies their landing requirements and the dubious benefits that MLS offers over ILS is currently not considered to be commensurate with the cost of retrofitting their aircraft with MLS avionics. In addition, the resulting MLS ground station hardware currently under development for military uses is complex and heavy (1000 pounds versus the desired 100 pounds) so that it does not satisfy those unique military tactical requirements that require extremely lightweight ground equipment.
What is required then in addition to the improved ILS system that is currently satisfying conventional civil requirements is a simple landing system that will satisfy unique civil and military landing needs that ILS will not satisfy. One unique civil application that ILS will not satisfy is to provide landing guidance to a rooftop heliport.
A second unique civil landing aid requirement is to provide landing guidance in a narrow valley like Aspen, Colo., where, despite many attempts by the FAA, it has not been possible to certify an ILS installation.
The unique military requirement of particular interest is that which requires an extremely lightweight ground station.
In addressing this problem of satisfying these unique civil and military requirements, it can be first noted that the use of microwave frequencies, instead of the low ILS frequencies, is mandatory for both unique civil and military applications. For example, in at least one unique civil application, such as a rooftop heliport, the low frequencies used in ILS will not provide the required guidance beams because such ILS guidance beams require a smooth ground plane, such as the airport surface, extending some 800 feet in front of the glide slope antenna (as noted previously) for proper beam formation. In the case of unique military applications, the light weight requirements mandates the use of microwaves for achieving small and lightweight antennas.
It can additionally noted that use of the MLS signal format is highly desirable since it is the only one that is currently universally accepted and/or likely to be universally accepted for the foreseeable future. A third consideration is to utilize some other guidance technique than one that requires the complex scanning beam mechanism of MLS.
2. Description of the Prior Art
Two separate landing systems, both operating at microwave frequencies, have evolved to meet these unique civil and military requirements. A first system is outlined in U.S. Pat. No. 4,635,064. That patent describes an MLS fixed beam, in contrast to a scanning beam landing system, which fixed beam landing system is much simpler than the conventional scanning beam MLS. The fixed beam MLS ground station radiates a signal that is compatible with the MLS signal format and, thus, can be received and utilized by an MLS-type receiver. This new and simpler C-band fixed beam MLS landing system has been given the acronym FBMLS (Fixed Beam Microwave Landing System) in contrast to the conventional Scanning Beam Microwave Landing System (SBMLS). FBMLS is suitable for all conventional civil airports, unique civil applications like rooftop heliports and conventional military fixed bases. Its primary advantages are its extreme simplicity due to its use of a fixed beam, in contrast to a scanning beam technology, and its compatibility with the MLS signal format.
This FBMLS system, in order to be fully compatible with MLS, operates in the MLS assigned C-band frequency allocation. For this reason, the FBMLS ground station is still relatively large and complex in terms of the desired military goal of 100 pounds, but still simpler and lighter than the SBMLS ground station, i.e., say 350 pounds versus the above-noted 1000 pounds.
A second landing system design, described in U.S. Pat. No. 4,429,312, focuses on the unique military requirement for a landing system ground station weighing 100 pounds, in 35 pound modules. This light weight characteristic permits the ground station to be readily transported by military personnel, such as paratroopers. Such a landing system with a lightweight ground station has been built and flight tested. It has been given the acronym PTAG (Portable Tactical Approach Guidance). The complete localizer and/or glide slope stations each weigh 35 pounds, for a total ground station weight of 70 pounds, i.e., it meets the 78 A.F. requirement for a 100 pound total weight in 35 pound modules. PTAG employs a higher microwave frequency than the MLS C-band frequency, which higher microwave frequency permits reduced antenna size, and hence, reduced weight. In addition, PTAG also employes simple fixed beam guidance technology similar to that used in FBMLS. The combined use of the higher microwave frequency, and simple fixed beam antenna technology has resulted in achieving the desired lightweight ground station. The system is being sold by Sundstrand (Data Control, Inc.) to the military under the PTAG trademark.